1. Field of the Invention
The present invention relates generally to methods and apparatus for providing FTTH (Fiber to the Home) bidirectional communications over a single optical fiber, and more specifically to NRZ (Non-Return to Zero) coded signals at a first frequency transmitted downstream and a Manchester coded signal at a second frequency modified by ON-OFF keying and transmitted upstream. The upstream and downstream coded signals are both used to modulate a carrier light wave having a selected wavelength of light, such as, for example, 1310 nanometers of light. The invention further relates to methods and apparatus for transmitting and recovering bursts of data with a minimal number of preamble bits and without first requiring phase lock.
2. Description of Related Art Including Information Disclosed Under 37 C.F.R 1.97 and 1.98
The communications industry is using more and more optical or light fibers in lieu of copper wire. Optical fibers have an extremely high bandwidth thereby allowing significantly more information than can be carried by a copper wire transmission line such as twisted pairs or coaxial cable.
Of course, modern telephone systems require bidirectional communications where each station or user on a communication channel can both transmit and receive. This is true, of course, whether using electrical wiring or optical fibers as the transmission medium. Early telephone communication systems solved this need by simply providing separate copper wires for carrying the communications in each direction, and this approach is still exclusively used in many locations and as part of the transmission path in many others. It is used to a greater degree as the signals get closer to the home or business end users. Although twisted pairs and coaxial cables are more likely to be used in distribution terminals close to the end user and homes, some modern telecommunication systems now use microwave and end-to-end optic fibers as the transmission mediums. In addition, various techniques are often used in optical transmission so that a signal optical fiber can carry more communication in both directions.
However, because of extremely high bandwidths available for use by an optical fiber, a single fiber is quite capable of carrying a great number of communications in both directions. One technique of optical transmission is WDM (Wavelength Divisional Multiplexing) which uses different wavelengths for different types of transmissions. Typical examples are the use of 1550 nanometers of light for TV signals transmission and 1310 nanometers of light for bidirectional telephony transmission.
It is noted that the term telephony is used rather than telephone to underscore the fact that communication transmission will include vocal telephone use but is not so limited. Typical telephony systems operate at a single frequency or wavelength of light which is divided into upstream and downstream carefully synchronized time windows for transmitting bursts of data. The use of such upstream and downstream synchronized windows is referred to as TDM (Time Division Multiplexing). This type of telephony systems use a single optical fiber and often may use only a single diode, for both converting electrical signals to optical signals and converting received optical signals to electrical signals.
However, as mentioned above, optical fibers have extremely high bandwidths and use of an optical fiber as a single bidirectional telephone channel is a very ineffective use of the fiber and, in fact, the available bandwidth of an optical fiber is what makes it possible to use two different and unrelated transmission techniques such as the transmission of bidirectional TDM telephone techniques at one wavelength, and the use of another technique, such as straightforward broadcasting of TV signals downstream at a second wavelength. Typically, two wavelengths regardless of the two techniques being used are combined by the use of WDM technology.
A major problem for the bidirectional telephony signals is light reflection typically occurring at optical connections or interfaces along the optical fiber, and in a worse case situation, the reflected energy may be interpreted as an actual signal transmission in the bidirectional communication. In addition, the typical use of NRZ (Non-return To Zero) coding and the need for increasing data transmission efficiency by using a minimum number of preamble bits are at odds with each other. Furthermore, clocking pulses and initial pulses of a transmitted signal are typically recovered by establishing a PLL (phase lock loop) by evaluating the time period between high to low and low to high transitions. Thus, since a consecutive string of “1”s or “0”s, using NRZ coding may result in the absence of any transition for an excessive period of time, the effect may be a shift in the timing of a data frame or “wander.” Therefore, a simple and straightforward technique to solve these problems would be of great value.
Therefore, a technique for transmitting bidirectional telephony signal bursts having minimal energy overlap, occurring from reflection and fast clocking recovery would allow the use of readily available hardware and make efficient and effective use of an optical fiber.